[PATCH 1/2] zhashfs, detect fill pattern and suppress blocks in output

Daniel Drake dsd at laptop.org
Tue Apr 26 14:25:34 EDT 2011

On 25 April 2011 20:37, James Cameron <quozl at laptop.org> wrote:
> No, because that would fail to detect blocks that have been read and
> then written unchanged.  While I don't know of any examples of that
> being done, it is still a possibility, either now or in the future.
> Using FIEMAP will place a kernel version dependency on zhashfs
> unnecessarily, since the task can be done with memcmp.
> It would also be more complex code to process the FIEMAP extents; more
> complex than memcmp.
> It would also require the output directory to be on a filesystem that
> supports sparse files.  (Attempting to create a sparse file using dd on
> a FAT filesystem results in a non-sparse file of the specified size ...
> a FIEMAP ioctl on this would show no holes).

I think all of those are safe assumptions to make, and that FIEMAP is
the more correct and efficient way of doing this.


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